1. Field of the Invention
The invention relates to lubricating base oil compositions with improved stability.
2. Description of the Prior Art
To be suitable for use as lubricating base oil, an oil should have, in addition to a certain minimum viscosity, a certain minimum viscosity index. Fractions from crude mineral oil having a sufficiently high viscosity for use as lubricating base oil, such as vacuum distillates and deasphalted vacuum residues, have, as a rule, a very low viscosity index, which is caused by the presence in these oils of a considerable amount of polyaromatic. A significant reduction of the polyaromatics content of the oils gives oils with a sufficiently high viscosity index to make them suitable for use as lubricating base oil. The desired reduction of the polyaromatics content may, in principle, be carried out in two different ways. The oil may be extracted with a selective solvent for polyaromatics, which produces a raffinate with the desired high viscosity index. Or the oil may be subjected to a catalytic hydrotreatment under such conditions that, among other things, polyaromatics are converted into compounds with a high viscosity index. The two treatments may also be combined. The starting material may first be subjected to a solvent extraction and the raffinate obtained may subsequently be subjected to a catalytic hydrotreatment. It is also possible to subject the starting material to a catalytic hydrotreatment first and then to subject the hydrotreated product to solvent extraction.
Processes for the preparation of lubricating base oils, in with polyaromatics are converted into valuable lubricating oil components by a catalytic hydrotreatment are preferred to processes in which the polyaromatics are removed from the oils by solvent extraction, since the former processes give a higher yield of lubricating base oils and the increase in the viscosity index is greater. These advantages also apply to the processes in which a combination of catalytic hydrotreatment and solvent extraction is used to reduce the polyaromatics content, the advantage being greater, of course, as a greater part of the polyaromatics is converted by means of a catalytic hydrotreatment. A drawback of the preparation of lubricating base oils from vacuum distillates and deasphalted vacuum residues by means of a catalytic hydrotreatment is that the catalytic hydrotreatment often has an unfavorable influence on the oxidation stability of the lubricating base oil prepared. This is connected with the removal of sulfur compounds from the oil occurring simultaneously with the conversion of the polyaromatics. As the catalytic hydrotreatment is carried out under more severe conditions in order to convert a greater part of the polyaromatics into valuable lubricating oil components, a lubricating base oil is obtained with a lower oxidation stability. As a rule, if the catalytic hydrotreatment is carried out under such conditions that it results in a reduction of the sulfur content of the oil of more than 90% (further designated as "severe catalytic hydrotreatment"), a lubricating base oil is obtained with an unacceptably low oxidation stability for practical use. This rule holds both for the preparation of lubricating base oils exclusively by a severe catalytic hydrotreatment and for the preparation of lubricating base oils by a combination of a severe catalytic hydrotreatment and a solvent extraction. The reduction of the sulfur content, as referred to herein, denotes: ##EQU1## where the %w S in product relates to the sulfur content of the hydrotreated product after components boiling below the initial boiling point of the feed have been boiled off. U.K. Patent Application No. 2,024,852 deals with the improvement of the oxidation stability of lubricating base oils prepared from vacuum distillate or deasphalted vacuum residue, using a severe catalytic hydrotreatment, which severe catalytic hydrotreatment was optionally carried out in combination with solvent extraction. In the '852 patent a small amount of a mildly hydrotreated deasphalted oil, a solvent refined vacuum distillate or a solvent refined deasphalted oil is added to lubricating base oils to improve oxidation stability. Severe catalytic hydrotreatment signifies a process in which a sulfur content reduction of more than 90% takes place, while a mild catalytic hydrotreatment is defined as a treatment in which a sulfur content reduction of less than 75% takes place. Although a considerable increase of the oxidation stability of the lubricating base oils can be obtained when the above-mentioned oils are used as additives, there remains a need for a further improvement of this property. A drawback of the application of the oils prepared either by solvent extraction or by mild catalytic hydrotreatment is that they sometimes have an adverse influence on the daylight stability of the lubricating base oil to which they are needed.